Atmospheric noise at the VLF/LF ranges is a highly impulsive and non-stationary phenomena over any time interval greater than ten to fifteen minutes. Because of the excellent propagation of energy in this frequency range, atmospheric noise in this range, commonly produced by lightning discharges, is characterized by high dynamic range impulses on the order of 60 db or more over that of receiver internally-generated-thermal noise.
The atmospheric noise, in general, is described as being a low amplitude Gaussian-type background noise in combination with that of the impulsive noise. Because of the nonstationary characteristics of the VLF/LF atmospheric noise, attempts have been made in the past to simulate its characteristics in such a manner that communication systems could be tested and evaluated with an acceptable degree of repeatability.
One such attempt manifested itself in the design of the "Simulated VLF/LF Noise Generator" of Paul Singer and Roger Cernius in U.S. Pat. No. 4,173,000. This design could be configured to produce a typical amplitude probability distribution of atmospheric noise within an expected dynamic range. As noteworthy as this design is, the control of the temporal characteristics of the impulse noise was preset and was not controllable independent of the amplitude probability distribution. A sophisticated noise analyzer for calibration, self-testing and setting up of signal-to-noise ratios for modem testing was required. A single amplitude probability distribution could be produced at a time for a given frequency from the programmable probability gate settings. Changes in the amplitude probability distributions require extensive hardware changes to the headers in the programmable probability and the set-up procedure involved an iterative trial and error process.
Thus, there is a continuing need in the state-of-the-art for a VLF/LF atmospheric noise generator being capable of generating a variety of amplitude probability distributions and discharge fine structure between discharges without requiring complicated and time consuming structural modifications to simulate atmospheric noise conditions that appear in different locations during changing seasons and for different portions of the VLF/LF electromagnetic spectrum.